Figure 1.9 depicts
arrivals that *moveout* from the source position with
offset.
The
*apparent* *velocity* *v*_{x}in the x direction of an arrival can be
computed by measuring the slope *v*_{x}=*dx*/*dt* of that
event's arrival time curve *t*(*x*), as shown on the RHS of Figure 1.8.
Arrivals with a large
apparent velocity and having a hyperbolic moveout curve
are
usually reflection events. For example, a 2-layer flat medium
with a first layer velocity of *v* and
an interface depth of *d*would see the traveltime equation
for the primary reflections as

where

Check out the *html* movie to see how plane
waves propagate with different apparent velocities
for different incidence angles.